Superficial benign cutaneous pigmented lesions such as lentigines, cafe' au lait and nevus spilus are commonly seen in dermatologic practice. A myriad of therapeutic modalities including liquid nitrogen, electrocautery and depigmenting chemicals have been used to remove them. Although widely used, none have succeeded in destroying the abnormal pigmented cells alone without damaging adjacent structures and producing adverse effects like hypopigmentation.
Over the last two decades, there have been several reports describing the removal of these pigmented lesions by a variety of lasers such as the excimer (351 nm), argon (488,514 nm), ruby (694 nm), Nd:YAG (1060 nm), and CO.sub.2 (10,600 nm) lasers. However, there has generally been damage to both pigmented and nonpigmented cells. Pigmented lesions treated by laser have included lentigines, nevi, melanomas, oral hypermelanosis of Peutz-Jeghers syndrome, the nevus of Ota, and a lentigo maligns. The pigment depth of these laser-treated lesions has also varied significantly, from superficial lentigines in the epidermis to lesions lying deep in the reticular dermis like the nevus of Ota.
Previous studies reporting "successful" removal of pigmented lesions have relied on clinical assessment rather than on histology and have used widely divergent wavelengths, pulse durations, energy densities and spotsizes. There has been no effort to define laser parameters necessary for optimal removal of pigmented lesions.
Melanin, an endogenous cutaneous pigment which is most concentrated in the basal layer of the epidermis, has an absorption spectrum that is highest in the ultraviolet range and gradually diminishes toward the infrared. Melanosomes, which are melanocyte-specific organelles, densely packed with melanin, are predominantly found within melanocytes. They vary in size according to their genetic origin; black skin typically containing larger melanosomes than lightly pigmented, white skin. Based on melanosome size, the calculated thermal relaxation time for these organelles is around 10 nsec. On the other hand, melanocytes are approximately 7 .mu.m in diameter, with thermal relaxation times around 1 .mu.sec. Thermal relaxation time in both instances is defined as the time taken for a structure to cool to 50 % of its peak temperature immediately after laser exposure.
Recent studies have applied the technique of selective photothermolysis to specifically destroy melanosomes using the XeF pulsed excimer in-vitro and the Q-switched ruby lasers in-vivo. Histologically, both these studies demonstrated melanosomal injury that was associated with disruption of melanocytes as well as melanin-containing basal keratinocytes. In addition, there was also evidence of follicular damage after exposure of pigmented guinea pig skin to the Q-switched ruby laser.